Bump gson from 2.8.6 to 2.8.9 in /prometheus-client

Bumps [gson](https://github.com/google/gson) from 2.8.6 to 2.8.9.
- [Release notes](https://github.com/google/gson/releases)
- [Changelog](https://github.com/google/gson/blob/master/CHANGELOG.md)
- [Commits](google/gson@gson-parent-2.8.6...gson-parent-2.8.9)

---
updated-dependencies:
- dependency-name: com.google.code.gson:gson
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

prometheus-client/pom.xml

@@ -35,7 +35,7 @@ limitations under the License.
         <dependency>
             <groupId>com.google.code.gson</groupId>
             <artifactId>gson</artifactId>
-            <version>2.8.6</version>
+            <version>2.8.9</version>
             <scope>compile</scope>
         </dependency>
         <dependency>